In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to charging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior to, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby.
Both D.C. and A.C. type corona devices are used to perform many of the above functions.
The conventional form of corona discharge devices for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating D.C. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an A.C. corona generating potential is applied to the conductive wire electrode and a D.C. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
Some of the problems with these devices are that the wires have a propensity for vibration, singing and sagging, especially when they are used for charging over a wide area and it is difficult to use them placed closed to the surface to be charged. Also, they are sensitive to breakage as well as difficult to mount.
Various approaches to answering these problems have been tried in the past. For example, U.S. Pat. No. 3,711,710 discloses a corona charging system wherein a plurality of ion discharging coronodes, made from thin conductive strips, which charge the surface of a receiving medium within a document reproduction machine. U.S. Pat. No. 3,959,690 is directed to a corona charging element for an electrophotographic reproduction machine that includes a coronode member in the form of a metal strip with peaks on one side thereof alternately positioned in the plane of one side of the strip and then the other with the peaks having the shape of a four sided pyramid and one side of the pyramid lying in the plane of the surface of the strip which charges a conductive photoreceptor surface by discharging an ion charging current directly onto the photoreceptor surface. An electrostatic precipitator apparatus is disclosed in U.S. Pat. No. 4,349,359 that includes an ion generating electrode in the form of a long twisted strip that charges the surface of collecting plates. U.S. Pat. No. 4,626,876 discloses a solid state corona discharger that includes a pair of parallel strip-shaped A.C. electrodes that discharge an ion charging current onto the surface of a photosensitive member. A particle charging apparatus is shown in U.S. Pat. No. 4,414,603 that includes a group of parallel, narrow strip-shape corona electrodes which collectively discharge an electric field to within a designated charging space when activated by an A.C. source voltage. Although these attempts at solving the above-mentioned charging problem have had some success, they have not been entirely satisfactory.